If you accept college B and get into college A later and prefer to attend college A you should simultaneously: 1) accept the offer from college A and send in a deposit; and 2) notify college B (in writing and hold onto a copy) that you are withdrawing from the college. Understand that you will likely lose your deposit paid to college B.
As others above have mentioned this does happen and it is not a problem.
What you cannot do is have deposits in at two different colleges.